On 2012-11-28 04:57, Mikulas Patocka wrote: > > > On Tue, 27 Nov 2012, Jens Axboe wrote: > >> On 2012-11-27 11:06, Jeff Chua wrote: >>> On Tue, Nov 27, 2012 at 3:38 PM, Jens Axboe <axboe@xxxxxxxxx> wrote: >>>> On 2012-11-27 06:57, Jeff Chua wrote: >>>>> On Sun, Nov 25, 2012 at 7:23 AM, Jeff Chua <jeff.chua.linux@xxxxxxxxx> wrote: >>>>>> On Sun, Nov 25, 2012 at 5:09 AM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >>>>>>> So it's better to slow down mount. >>>>>> >>>>>> I am quite proud of the linux boot time pitting against other OS. Even >>>>>> with 10 partitions. Linux can boot up in just a few seconds, but now >>>>>> you're saying that we need to do this semaphore check at boot up. By >>>>>> doing so, it's inducing additional 4 seconds during boot up. >>>>> >>>>> By the way, I'm using a pretty fast SSD (Samsung PM830) and fast CPU >>>>> (2.8GHz). I wonder if those on slower hard disk or slower CPU, what >>>>> kind of degradation would this cause or just the same? >>>> >>>> It'd likely be the same slow down time wise, but as a percentage it >>>> would appear smaller on a slower disk. >>>> >>>> Could you please test Mikulas' suggestion of changing >>>> synchronize_sched() in include/linux/percpu-rwsem.h to >>>> synchronize_sched_expedited()? >>> >>> Tested. It seems as fast as before, but may be a "tick" slower. Just >>> perception. I was getting pretty much 0.012s with everything reverted. >>> With synchronize_sched_expedited(), it seems to be 0.012s ~ 0.013s. >>> So, it's good. >> >> Excellent >> >>>> linux-next also has a re-write of the per-cpu rw sems, out of Andrews >>>> tree. It would be a good data point it you could test that, too. >>> >>> Tested. It's slower. 0.350s. But still faster than 0.500s without the patch. >> >> Makes sense, it's 2 synchronize_sched() instead of 3. So it doesn't fix >> the real issue, which is having to do synchronize_sched() in the first >> place. >> >>> # time mount /dev/sda1 /mnt; sync; sync; umount /mnt >>> >>> >>> So, here's the comparison ... >>> >>> 0.500s 3.7.0-rc7 >>> 0.168s 3.7.0-rc2 >>> 0.012s 3.6.0 >>> 0.013s 3.7.0-rc7 + synchronize_sched_expedited() >>> 0.350s 3.7.0-rc7 + Oleg's patch. >> >> I wonder how many of them are due to changing to the same block size. >> Does the below patch make a difference? > > This patch is wrong because you must check if the device is mapped while > holding bdev->bd_block_size_semaphore (because > bdev->bd_block_size_semaphore prevents new mappings from being created) No it doesn't. If you read the patch, that was moved to i_mmap_mutex. > I'm sending another patch that has the same effect. > > > Note that ext[234] filesystems set blocksize to 1024 temporarily during > mount, so it doesn't help much (it only helps for other filesystems, such > as jfs). For ext[234], you have a device with default block size 4096, the > filesystem sets block size to 1024 during mount, reads the super block and > sets it back to 4096. That is true, hence I was hesitant to think it'll actually help. In any case, basically any block device will have at least one blocksize transitioned when being mounted for the first time. I wonder if we just shouldn't default to having a 4kb soft block size to avoid that one, though it is working around the issue to some degree. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
